HUDs in vehicles usually consist of an imaging system and a mirror system with 2 or more mirrors, which are all located in the dashboard of the vehicle. With an HUD, the image information emitted by an imaging system is made accessible to an observer, e.g. the driver of a motor vehicle or the pilot of an aeroplane, as a virtual image in their visual field. The size of the image generated by the HUD within the visual field of the observer is usually known as the field of view (FOV). The area on the image side at the observer which is at maximum illuminated by the imaging system is known as the eyebox. The desired size of the eyebox and the FOV define the necessary étendue which must be provided by the imaging system.
In most cases, the visibility of the image information shown is limited to a certain spatial area within the vehicle, so that the observer should be located at least with one eye within the head motion box (HMB) defined by display system used. The HMB has usually a rectangular base area with a size of approx. 220×80 mm2. The HMB is thus related to the observer and indicates the permissible spatial range within which the observer can at least detect a part of the image information presented by the HUD with one eye. On the other hand, the eyebox is more related to the imaging system and is based on the maximum aperture angle of the image in a direction towards the observer. For current HUDs, the HMB and the eyebox do not or not significantly differ from each other in general.
The image sensor serves to display and irradiate image information. A corresponding individual component is described as an imaging system. This can for example be a TFT/LCD display or a Pico projector. The imaging system usually comprises a so-called “imager” (image generator) for displaying the image information. Possible imagers usually take the form of TFT/LCD panels, DMD, LCoS, MEMS or similar. Preferably, the imaging system can be a Pico projector with DMD/LCoS as an imager or also a simple TFT panel. If a TFT panel is used, the image information irradiated by the TFT panel is made accessible to the observer directly as a virtual image in the visual field. If a Pico projector is used, this first generates an interim image on an additional interim screen, wherein this interim image essentially takes on the function of a TFT. The interim image is then usually displayed by a mirror system in the eyebox of the observer.
From a given étendue request (based on a desired size of the eyebox and the FOV) and the precise irradiation properties of a known imaging system, the required size of the imaging system and the irradiation angle to be used can be determined. From this, the minimum size of a TFT panel to be used or of the interim screen when a Pico projector is used is determined.
If a TFT panel is used, image information that is irradiated as a real image of the TFT panel is propagated with the aid of mirrors to a virtual image in front of an observer, with motor vehicles in front of the windscreen, for example. Here, reference can in general be made to a single-step display (imaging system→virtual image).
If a Pico projector is used as an imaging system, the étendue already supplied by the imager, such as a DMD or LCoS, for example, is usually not sufficient. For this reason, it must be enlarged accordingly. Typically, this occurs with the aid of a diffusing panel which is positioned on the plane of the interim image. By enlarging the angle, the diffusing panel also enlarges the étendue of the real interim image on the interim image plane. This interim image with enlarged étendue can now also be propagated to a virtual image within the visual field of an observer with the aid of mirrors. Here, reference can be made to a dual-step display (imaging system→real interim image→virtual image).
In motor vehicles, the HUD has to date been located solely in the dashboard. Here, construction space of approx. 5 l is required for an FOV of 6°×2°. However, in most cases, the available construction space can only be influenced to a limited degree. The size of the first mirror (from the perspective of the virtual image) is essentially determined by the FOV, the angle of the windscreen, and the size of the eyebox. The curve of the windscreen, the optical path from the eye of the observer across the windscreen to the first mirror, and the position of the imaging system within the dashboard are also influential. With the exception of the position of the imaging system, these values are essentially prespecified. However, different positions of the imaging system also require slightly different tilting angles of the mirror to the main beam. As a result, the mirror size is also marginally influenced. The first optical component that can be influenced from the perspective of the driver is thus the first mirror mentioned above.
Future HUDs should support Augmented Reality and thus extend reality by showing additional information or displays. In particular, here, it should be possible to project location-accurate images to the observer, which interact directly and immediately with the visible environment of the observer. Thus, reality can be virtually extended as required, depending on the demands of the current vehicle situation.
In order to realise an Augmented Reality HUD (AR-HUD) in vehicles, a particularly large FOV is required (FOV 10°×6° or larger). Due to the large FOV, significantly larger mirrors for deflecting the beam are also required. These must also be housed in the dashboard and, as a result, require additional construction space (currently more than 15 liters). In the case of an enlargement of the FOV, however, it must be noted that for a fixed imaging system, the brightness of the displayed image is in general reduced. Thus, in order to maintain the brightness of the image and to increase the size of the FOV, in the case of a HUD with an unchanged size of the eyebox, a corresponding enlargement of the imaging system and of the associated display system must also be planned in addition to an enlargement of the deflection device.
Particularly in the dashboard of a motor vehicle, however, there is an increasing lack of space, since the steering column, air conditioning unit, control panel cross-member, windscreen wiper mechanics, windscreen ventilation, a large portion of the vehicle electronics and much more also need to be housed here. The lack of space in the dashboard thus limits the maximum possible size of the FOVs. The car industry, as well as other vehicle manufacturers, is therefore interested in particularly space-saving solutions. For this reason, larger FOVs can only be realised, and the Augmented Reality experience significantly improved, through the corresponding savings in the construction space required.
Vehicle manufacturers are also increasingly in favour of saving electrical energy. With motor vehicles, this is clearly reflected in particular by the increasing use of electricity-saving LEDs for external lighting. If a vehicle uses less energy, the light machine is less of a burden on the engine, which in turn helps to save fuel. Energy savings are also gaining in importance with regard to the achievement of international climate goals. The use of HUDs in vehicles adds a further electric consumer which consumes not inconsiderable levels of power. With current HUDs, however, a large portion of the energy required to generate images is lost unused. On the one hand, with a large eyebox (220×80 mm2), it is often the case that only a very low portion of the energy irradiated from the imaging system contributes to the image information perceived by the observer (up to 0.3%). On the other, with TFT panels or LCoS, often considerable polarisation losses occur, since unpolarised light sources, such as LEDs, are usually used. Additionally, with the dual step display, a great deal of light is lost on the diffusing panel, since the angle spectrum generated is far greater than the area which can be used to display the virtual image.
It is therefore the object of the present invention to provide a system and a method for operating an HUD which overcome the described disadvantages of the prior art, and which make it possible to enable corresponding display applications, even with limited construction space with the largest possible FOV with high energy efficiency. In particular, the increased use of Augmented Reality in vehicles should be enabled as a result.